I personally think there's many reasons to Su35 purchase, but mainly for tech transfers. But hold your horses before you get angry and attack me with your keyboard
D), let me explain.
By tech transfer, I don't mean hand down or help to start, like China is to Pakistan, but more like UK/US in JSF program, I know a little more about this, you know a little more about that. China did advance in leaps and bounds in recent years, but decades of Soviet expertise is still useful. There must be some techs and even small tips from decades of sheer experiences, being used in Su35 that China still can learn from. In this case, many of those things can be transferred onto domestic models, like using airframe and avionic tech for J20/J16, engine tech from 117S for WS10/15, maybe even weapon tech for domestic Chinese weapons. It doesn't mean that China has to copy the whole engine or solely rely on Russians for engine production.
Another reason would be the relative low cost to its price. Currently Russian economy is hurting; and really hurting. So Russia would have to and it did, sell the plane at a very low price. According to RT, they're priced at only 83 million each, which is pretty cheap for a plane of this caliber. This is not much more than making them yourself, so why not take advantage of it. It's almost like buying the planes at cost and getting the tech transfer as a free bonus.
The third reason would probably be a geopolitical one. China wants to help Russia financially, so it could keep attracting all the heat from the West, instead of giving the West time to think about how to contain China.
That's just my two cents.
